Generally MOS circuits are provided with a substrate bias circuit because of the advantage obtainable from the substrate bias voltage. That is, if a negative VBB voltage is applied to the substrate of an NMOS LS1 or VLSI chip, then the sensitivity of the threshold voltage due to the body effect can be lowered, the punch-through voltage can be increased, and the ratio between the diffusion and the capacitance of the substrate can be lowered without reduction of the doping of the substrate. Further, the subthreshold leakage for a clocked depletion transistor can be reduced, and the chip can be protected from the forward biasing. Here, the forward biasing of the substrate is indicative of a voltage undershoot generated at an input terminal commonly shared by the TTL peripheral circuits. (Refer to "The Design and Analysis of VLST Circuits;" Lance A. Glasser and Daniel W. Dobberpuhl, 1985.)
The usual substrate bias circuits generating a negative bias voltage emits AC signals having a required frequency through an oscillator, and these AC signals are amplified by means of a driver. Further, the AC signals amplified by the driver are supplied to a charge pump. Therefore, the charge pump pumps the charges from the substrate to a ground node, so that the substrate should become negatively biased.
However, there arises a problem when a substrate bias circuit is installed in a semiconductor chip, the problem being such that the substrate bias circuit has to be constantly driven even in a waiting state of the semiconductor chip, and therefore, the standby current is increased. Further, if the substrate bias circuit is varied due to a noise or due to a variation of the power source, then the various electric parameters of the semiconductor device are also varied.
Further, according as the density of a semiconductor chip is increased, two or more substrate bias circuits are provided in a single chip in order to supply a stable substrate bias voltage. However, in such a case, the standby current can be increased to a greater extent.